Method for stretching cellulose ester yarn



Patented Sept. 25, 1945 METHOD FOR STBETCHING CELLULOSE ESTER YARNRollin F. Conaway,

E. I. du Pont de Nemour Wilmington, DeL, assignmto s & Company,Wilmington, Del., a corporation of Delaware No Drawing. ApplicationApril 8, 1941, Serial No. 387,550

9 Claims.

This invention relates to the treatment of preformed filaments, yarns,threads, and ribbons of artificial thermoplastic material. Moreparticularly it relates to a method for the thermal stretching offilaments, yarns, threads, and ribbons of thermoplastic cellulosederivatives, whereby to increase the tenacity or tensile strengththereof.

In the stretching of preformed fibers, one of the importantprerequisites and most difficult objectives to accomplish is renderingthe article uniformly plastic prior to stretching, since smalldifferences in plasticity produce large differences in the chemical andphysical characteristics of the stretched article. The generalprocedures employed for stretching preformed fibers may be classified asfollows: (1) solvent swelling methods; (2) combined solvent swelling andthermal procedures; and (3) thermal processes. The thermal stretchingprocedure has many inherent advantages over the other processes, but inall of the variations of this procedure heretofore disclosed, heatingand stretching has taken place in air, with the result that thefilaments, etc. are not plasticized uniformly and the quality of thestretched product suffers accordingly.

An object of this invention therefore is to uniformly plasticize apreformed filament, yarn, thread or ribbon of. artificial thermoplasticmaterial, by heat treatment alone, and to stretch the same while it isundergoing the heat treatment. A further object is to provide a methodfor uniformly heat-plasticizing and stretching preformed filaments,yarns, threads, or ribbons of thermoplastic cellulose derivatives, inthe substantial absence of air and media which would exert anappreciable swelling or solvent action on the cellulose derivative.These and other objects will more clearly appear hereinafter.

These objects are accomplished by the followme invention which, broadlystated, comprises softening and stretching filaments, yarns, threads andribbons of artificial thermoplastic materials in a liquid mediummaintained at a temperature above the softening point of thethermoplastic materials, which medium is relatively nonvolatile, stable,and substantially inert toward the artificial materials at thetemperature of treatment, i. e. which does not appreciably swell ordissolve them or react chemically with them.

In its preferred form the process consists essentially in passing yarnconsisting of continuous thermoplastic filaments of a cellulosederivative at a uniform rate into a liquid bath maintained at atemperature sufiiciently high to render the sufilcient tension to the tostretch it to the deyarn plastic and applying thermally softened yarnsired extent, Preferably the yarn is wet or moistened with water priorto its passage through the bath, for I have found yarns so treated yieldstretched yarns of superior tensile strength. The process is carried outby passing the yarn under tension over a positively-driven feed roll andeither horizontally or vertically through the hot liquid bath and onto apositively-driven wind-up bobbin. The extent of stretching is determinedby the relative speeds of the feed and wind-up rolls. The baths can beheated by any of the standard procedures that are subject to accuratecontrol, such as electrical units, steam, or vapors of boiling liquids.An antisticking finish, such as that described in copending applicationSerial No. 387,551 filed April 8, 1941, can be applied to the yarn in aseparate operation or by inserting a size roll or other suitable sizingequipment in the process before the yarn reaches the hot liquid bath.

The substances employed as the heating media may consist of anysubstance which is liquid and substantially nonvolatile at thetemperature required to soften the yarn, and which is substantiallyinert toward the yarn, i. e. does not swell, dissolve, or reactchemically with the yarn. They may be classified broadly into two types,namely those that wet the yarn and those that do not wet or adhere tothe yarn. Potassium thiocyanate, which melts at 172 C. to form a clear,

stable liquid, is an example of a suitable liquid heating medium thatwets the yarn, whereas fusible metals, such as Wood's metal and ordinarysolder (50:50 lead-tin), are examples of liquid heating media that donot wet the yarn.

The temperature of the liquid heating medium depends on the compositionof the yarn, the effective length of the bath, the denier of the yarn,and the extent and rate of stretching. In the stretching of thecellulose acetate textile yarns, at speeds of -500 ft./min., bathtemperatures in the order of 220-260 C. are required.

The methods for processing the stretched yarns depend to an appreciableextent on the type of heating medium employed. When the stretched yarnis coated with the heating medium, such as with potassium thiocyanate,the yarns are usually washed and dried prior to twisting and coning,whereas with the molten metal media that do not adhere to the yarn, theyarn can be processed in regular textile equipment without washing andthe various finishes removed from the yarn in the regular boil-offoperations prior to dyeing.

The following examples further illustrate the process.

Example I This example illustrates the stretching of cellulose acetateyarn with a potassium thiocyanate bath.

A dry 300 denier-100 filament cellulose acetate yarn was stretched 500%by passing the yarn 20 ft./min. and a wind-up speed of 100 ft./min.'I'he'yarn was passed through the molten bath maintained at atemperature of 238 C. in which the contact length of the yarn with thebath was approximately 6 inches. The yarn passed through the bath in ahorizontal manner by means of suitable guide rolls. The resultingpurified 60 denier-100 filament yarn possessed 2.

Example II This example illustrates the stretching of water-wetcellulose acetate yarn in molten potassium thiocyanate.

A 300 denier-100 filament cellulose acetate yarn was wet with stretchingthe passed into a molten potassium thiocyanate bath under a tenture ofthe bath' was 238 length of the yarn with the bath was 10 inches. Theresulting purified 60 denier-100 filament yarn possessed a tenacity of3.5 g./den. and an elongation of 6%. The maximum variation in C. and thecontact ing media is not obtained ing units.

melting alloy (61 C.) containing 12.5% tin, 25% lead, 12.5% cadmium, and50% bismuth.

A 300 denier-100 filament cellulose acetate yarn. The

C., at a speed of 5.5 ft./mm., and a wind-up speed of 33 the yarn withThe resulting 50 denierfilament stretched yarn was f from stuckfilaments and possessed a tenacity 3.4 g./den., an elongation of 4%, andunifo physical and chemical properties.

Example IV This example illustrates the stretching of 01 lulose acetateyarn in a molten solder (50: tin-lead) bath.

A 300 denier-100 filament cellulose aceta with an aqueous solution of lpossessed a tenacity of 3.3 g./den. and an elonga tion of 5.5%.

In contrast, the same type of cellulose acetate yam was sized under thesame conditions witl parts of sodium chloride, by passing the yarn over./min. One

and 86 parts of water a size roll at a linear namely, a bath length of14 inches, a bath tem- C., a feed speed of 20 It./min., a wind-up speed01 ft./min., and a stretching tension of 7 grams. The 76 denier-66filament stretched yarn obtained by stretching the moist, sized yarnpossessed of 3.1 g./den. and an elongation This shows as the heatingmedia, the tenacity of the stretched yarn is improved if the originalyarn is moist or waterwet when passed into the hot liquid medium.

Example VI This example illustrates the stretching of cellulose acetateyarn possessing high yarn and filament deniers in molten metal baths.

"-A 2400 denier-120 filament cellulose acetate yarn sized with anaqueous solution, described in copending application Serial No. 3 7,551,containing 2 parts of saponin, 10 parts of sodium chloride, and 88 partsof water was stretched 1600% by passing the moist yarn into a solderbath at a temperature of 240 C., under a stretching tension of 20 grams,a feed speed of 8 ft./min., a wind-up speed of 128 ft. /min. and a bathlength of 21 inches. The resulting 150 denier-120 filament stretchedyarn possessed a tenacity of 2.4 g./den., an elongation of 4%, a softhand indicating the absence of stuck filaments, and uniform chemical andphysical properties. The original 2400 denier yarn possessed a tenacityof 1.2 g. /den. and an elongation of 50%.

Example VII This example illustrates the stretching of cellulose acetateyarn in molten tin as the heating medium.

A 660 denier-66 filament cellulose acetate yarn sized with an aqueousantisticking finish, described in copending application Serial No.387,551, containing 2 parts of saponin, 10 parts of sodium chloride, and88 parts of water, was stretched 1025% in a vertical bath of molten tin.The heating bath was 2 inches in diameter and 8 inches in depth so thatthe contact length 01 the yarn with the molten tin was approximately 15inches, since the yarn passed over a metal roller guide in the bottom ofthe bath. This bath was surrounded'by a metal jacket and was heateduniformly by Dowtherm vapor. The moist, sized yarn was stretched in themolten tin at a feed speed of 20 ft./min., a wind-up speed of 205 ft./min., a stretching tension of 10 grams, and a bath temperature of 242 C.The resulting 65 denier-66 filament stretched yarn possessed a tenacityof 3.0 g./den. and an elongation of 4%.

Example VIII stretching of celin a molten solder A 150 denier-38filament mixed ester yarn prepared from cellulose acetate propionatecontaining approximately 2.5% of combined propionic acid and 52% ofcombined acetic acid was sized with an aqueous solution, described incopending application Serial No. 387,551, containing 2 part of saponin,10 parts of sodium chloride, and 88 parts of water. The wet, sized yarnwas passed into a molten solder bath maintained at a temperature of 240C., at a feed speed of 22 ft./min. and a wind-up speed of 132 ft./min.The heat-softened yarn was stretched 600% in the solder bath under astretching tension of 8 grams. The resulting stretched yarn possessed atenacity of 3.0 g./den., an elongation of a soft pleasant handindicating the absence of stuck filaments, and uniform deniercharacteristics.

Example IX This example illustrates the stretching of delusteredcellulose acetate yarn in a molten metal bath.

A 300 denier-100'filament cellulose acetate yarn delustered withtitanium dioxide pigment was sized with an aqueous solution, describedin copending application Serial No. 387,551, constaining 2 parts ofsaponin, 10 parts of sodium chloride, and 88 parts of water, by passingthe yarn over a size roll at a linear speed of 22 ft./min. The resultingmoist, sized yarn was stretched 600% by passing the yarn into a hotmolten solder bath maintained at a temperature of 240 C. Th stretchingtension on the yarn was 8 grams, the wind-up speed was 132 ft./min., andthe contact length of the yarn with the bath was 12 inches. Theresulting delustered denier stretched yarn possessed a tenacity of 3.0g./den. and an elongation of 4%- The invention has been illustrated withparticular reference to thermoplastic organic derivatives of cellulose,such as cellulose acetate, nitrate, propionate, butyrate, acetatepropionate, acetate benzoate, ethyl cellulose, benzyl cellulose, ethyllauryl cellulose, and ethyl celluose acetate; it is to be understood,however, that the process is applicable to thermoplastic artificialfilamentforming material in general, including vinyl resins, polyamides,polyesters, polyalcohols, etc.

The invention is further susceptible to a wide variation in materialsand conditions not specifically delineated in the examples. Thus theyarn may either be dry (0% to 5% moisture) or contain a substantialquantity of water (waterwet). Stretched yarns with improved physicalproperties are obtained when the original yarn contains between 5% andof water. The preferred amount of water associated with the yarns isbetween 5% and 50% based on the dry weight of the yarn. The yarn maycontain small amounts of additional materials such as delusterants,resins, or sizes ordinarily employed to improve the quality orfacilitate handling of the yarn.

Any of the commercial methods, such as by positively-driven rolls andbobbins for passing yarn continuously'from one point to another, may beemployed for passing the yarn continuously and uniformly through theheated liquid baths. The temperature of the heating bath should beuniform and be controlled very accurately, such as to within :1" C. ofthe desired temperature. The baths may be heated by any of the standardmethods, such as by steam, electrical units, or heated vapors.

The heating medium may be any substance that is a mobile, stable, andrelatively nonvolatile liquid at the desired stretching temperature anddoes not have a solvent or swelling action on the yarn at thistemperature. In reference to each of these limitations, it is desirablethat the liquid be mobile at the stretching temperature so that the yarncan be passed through the bath easily. The liquid bath should berelatively stable to decomposition by prolonged thermal treatment athigh temperatures and to the action of steam or any of the components inthe finishes that may be applied to the yarn. The bath should be neutralin order that the cellulos derivative will not be degraded or decomposedby the stretching treatment and should be relatively nonvolatile attemperatures up to 275 C. so that the bath can be operated atatmospheric pressure without an appreciable loss of the heating mediumby evaporation. A most important requisite of the liquid bath is thatthe liquid heating medium should not have an appreciable swelling orsolvent action on the yarn.

The substances comprising the heating medium may differ widely in othercharacteristics, such as general solubility, melting points, andspecific gravity. Many of the substances fulfilling the requirements aresolids at room temperature, such as potassium thiocyanate which melts at172 C., Wood's metal at 61 0., ordinary solder at 180 C., and tin at 232C. At stretching temperatures above 200 C. the low melting inorganicsalts, such as potassium thiocyanate and eutectic mixtures of sodiumnitrate with other inorganic nitrates and the fusible metals or alloysare the most satisfactory media.

The stretching conditions may. vary widely and are determined largely bythe composition and denie'r of the original yarn or article, the lengthof the heatlng bath, the stretching tension, and the rate and extent ofstretching. In general, the longer the bath length the higher is thestretching tension that can be obtained, and the slower the rate ofstretching the lower is the temperature at which the bath can beoperated. The bath length can vary from a few inches to several feet inlength and one yarn or a number of yarns can be stretched simultaneouslyin the same bath. In the case of cellulose acetate yarn, the bathtemperature can vary from approximately 200 C. with long baths, smallyarn, and slow rates of stretching to 300 C. or more for short baths,large yarn, and high rates of stretching. The stretching tensions canvary rather widely, such as from 1 gram to 50 grams depending on theyarn, stretching temperature, etc. Likewise, the extent and rate ofstretching can vary widely. In general, it is ciable improvement intenacity, and higher extents of stretching, such as in the order of500400095, are preferable. The extent of stretching is determinedprimarily by the char a continuous or step-wise manner and specialfinishes may be applied to the yarn prior to stretching or to thestretched yarn.

The stretched yarn may be processed in a number of ways. It is generallynecessary to in the regular boil-off operation prior to dyeing. Ingeneral, the stretched yarns can be processed in the same manner asregular textile yarns.

yarns of a physical and chemical uniformity not heretofore obtainable.

I claim:

silestrength thereof.

2. The process which comprises passing preformed water-wet filaments,yarns, threads and state whereby thereof.

the softened state.

acetate containing from about 5% to about 100% by weight of water basedon the weight of dry cellulose acetate into a bath consisting of 50%lead and 50% tin maintained at a. temperature 5 sufliciently high tosoften the filaments, and

stretching said filaments to the desired degree while in the softenedstate.

ROILIN F. CONAWAY.

